Format all the things (#991)

* Format Javascript and CSS files with `prettier`: `prettier --write 'CTFd/themes/**/*'`
* Format Python with `black`: `black CTFd` & `black tests`
* Travis now uses xenial instead of trusty.

tests/utils/test_validators.py

@@ -3,9 +3,9 @@ from marshmallow import ValidationError
 
 
 def test_validate_country_code():
-    assert validate_country_code('') is None
+    assert validate_country_code("") is None
     # TODO: This looks poor, when everything moves to pytest we should remove exception catches like this.
     try:
-        validate_country_code('ZZ')
+        validate_country_code("ZZ")
     except ValidationError:
         pass